1. Field of the Invention
The present invention relates to casting methods. More specifically, the present invention provides an apparatus and method of unidirectionally solidifying castings to provide a uniform solidification rate, thereby providing a casting having a uniform microstructure and lower internal stresses.
2. Description of the Related Art
Various methods of directional solidification of castings within the mold have been attempted in an effort to improve the properties of castings.
An example of a presently available directional solidification method includes U.S. Pat. No. 4,210,193, issued to M. Rühle on Jul. 1, 1980, disclosing a method of producing an aluminum silicone casting. The molten material is poured into a mold having a bottom formed by a tin plate. A stream of water is applied to the bottom of the tin plate, and a thermocouple inserted through the tin plate into the casting is used to monitor the temperature of the casting, and thereby properly control the cooling stream. Cooling is stopped when the temperature in the bottom portion of the mold falls from 575° F. to 475° F., until heat from the surrounding melt increases this region to 540° F. When the aluminum silicone alloy is removed from the mold, the tin plate has become a part of the casting. The result is a fine grain structure in the lower portion of the casting. This method fails to produce a uniform structure with low stresses, and would likely result in waste due to the necessity of cutting away the tin plate if it is not to form a part of the final casting.
U.S. Pat. No. 4,585,047, issued to H. Kawai et al. on Apr. 29, 1986, discloses an apparatus for cooling molten metal within a mold. The apparatus includes a pipe within the mold through which a cooling liquid is passed. The pipe is located in a lower portion of the mold, resulting in directional solidification of the metal from the bottom of the mold to the top. Once the casting is solidified, the excess portion of the casting is cut away from the casting, and then melted away from the pipe so that the pipe can be reused. The necessity of cutting away the portion of the casting surrounding the pipe results in added manufacturing steps and waste. The apparatus further fails to provide for a uniform structure within the casting or the low stresses within the casting that would result from a directional solidification.
U.S. Pat. No. 4,969,502, issued to Eric L. Mawer on Nov. 13, 1990, discloses an apparatus for casting of metals. The apparatus includes an elongated pouring device structured to pour molten metal against a vertical plate, thereby dissipating the energy of the flowing molten metal. Alternatively, a pair of elongated pouring devices are used to pour molten metal towards each other so that the interaction of the two strains of metal flowing towards each other dissipates the energy of the metal. The result is a reduced wave action within the mold, so that the cooled casting has a more uniform thickness. The apparatus fails to provide for a uniform structure within the casting. It also fails to provide low stresses within the casting.
U.S. Pat. No. 5,020,583, issued to M. K. Aghajanian et al. on Jun. 4, 1991, describes the directional solidification of metal matrix composites. The method includes placing a metal ingot above a mass of filler material and then melting the metal so that the metal infiltrates the filler material. The metal may be alloyed with infiltration enhancers such as magnesium, and the heating may be done within a nitrogen gas environment to further facilitate infiltration. After infiltration, the resulting metal matrix is cooled by placing it on top of a heat sink, with insulation placed around the cooling metal matrix, thereby resulting in directional solidification of the molten alloy. This patent fails to provide for control of the rate of solidification, for a uniform structure within the castin, or for low stresses within the casting.
U.S. Pat. No. 5,074,353, issued to A. Ohno on Dec. 24, 1991, discloses an apparatus and method for horizontal continuous casting of metal. The system includes a holding furnace connected to a hot mold having an open section at its inlet end. Heating elements around the sides and bottom of the hot mold heat the mold to a temperature that is at least the solidification temperature of the casting metal. A cooling spray is applied to the top of the hot mold. A dummy member secured between upper and lower pinch rollers is reciprocated into and out of the outlet end of the mold to draw out the metal as it is solidified. The method of this patent is likely to result in waste due to the need to separate the casting from the dummy metal. The apparatus further fails to provide for a uniform structure within the casting or the low stresses within the casting that would result from a directional solidification.
Accordingly, there is a need for an improved apparatus and method of unidirectional solidifying of casting, providing for a relatively uniform, controlled cooling rate. Such a method would result in greater uniformity within the crystal structure of the casting, with lower stresses within the casting, and a reduced tendency towards cracking.